1. Field of the Invention
This invention relates in general to a method for protecting a phase lock loop (PLL) in an optical system. More particularly, the invention relates to a method for protecting a phase lock loop being capable of preventing the optical system from malfunctions caused by abnormally performing frequency locking and adjustment operations when error data is read. Additionally, the disclosed method is adapted to an optical system under a constant angular velocity (CAV) modulation mode that the system can reduce the operating time when adjusting system clock frequencies under seeking mode.
2. Description of Related Art
Many optical systems, such as optical drives, optical writers, digital optical drives or high-speed readable/rewritable optical drives, are subsequently commercialized as technologies being highly developed. An optical disc is a known portable storage media having a storage capacity larger than a floppy disk, which makes the optical disc becomes widely used media for storing information now. Basically, pits and lands recorded on the spiral tracks of the optical disc are used to represent data stored thereon. In general, the optical system has to obey a certain rule for ensuring the accuracy of the accessed data in order to prevent itself from reading error data or from failing to access correct data due to misalignment to the spiral tracks under following mode. Conventionally, the waveform lengths of the EFM signal should be constrained between three system-clock periods (3T) and eleven system-clock periods (11T), while a PLL circuit is a widely used one in the optical system for locking the frequency of the system clock so as to achieve the purpose of controlling the waveform lengths of the EFM signal.
According to the conventional technology for locking the waveform lengths of the EFM signal, when the accumulated times that the waveform lengths of the EFM signal within a detecting window longer than 11 system clock periods has reached a preset threshold, the frequency locked by the PLL circuit will be decreased in the next detecting window. In contrary, when there is no EFM signal having waveform length exceeding 11T, the frequency locked by the PLL circuit will be increased in the next detecting window. Therefore, all data stored in the optical disc can be correctly accessed since the system clock frequencies can be properly adjusted. Conventionally, the detecting window can be 1˜8 frames and each frame contains 588 system clock periods (588T).
For example, the system clock SYS_CLK in FIG. 1 is generated and used for detecting the EFM signal in the optical system such as an optical drive, while the EFM signal is described as series EFM_DATA_A˜EFM_DATA_C in FIG.1. Additionally, the preset threshold is assumed to be three (3), that is, the frequency of the system clock SYS_CLK has to be decreased when the waveform lengths of the EFM signal longer than 11T has reached three times (i.e., the preset threshold). In the following description, the waveform lengths of the EFM signal before Ti is assumed to be shorter than 11T.
In FIG. 1, when the system clock SYS_CLK samples the EFM_DATA_A series, assume that the EFM signal having a waveform length longer than 11T occurs in consecutive system clock periods Ti+1˜Ti+14. Next, the system clock SYS_CLK samples the EFM_DATA_A series and then detects the EFM waveform lengths in Tj+1˜Tj+12 (indicated by numeral 102B) and Tk+2˜Tk+16 (indicated by numeral 102C) are longer than 11T, wherein the waveform lengths in 102B and 102C are 12T and 15T respectively. Because the accumulated times regarding the waveform lengths of the EFM signal longer than 11T has reached three times (indicated by 102A, 102B and 102C), the optical system will decrease the frequency of SYS_CLK in the next detecting window. On the other hand, there is no detected waveform length in the EFM_DATA_B series being longer than 11T when the system clock SYS_CLK samples the EFM_DATA_B waveform. For example, the waveform lengths in Ti+3˜Ti+13 (indicated by 104A), Tj+1˜Tj+5 (indicated by 104B), Tk+1˜Tk+9 (indicated by 104D) and Tk+9˜Tk+16 (indicated by 104C) are 10T, 5T, 8T, 9T and 8T, respectively. Because there is no waveform length longer than 11T in the EFM_DATA_B series, the optical system will increase the frequency of SYS_CLK in the next detecting window. Continuing the above processes, the frequency locked by the PLL is adjusted when the EFM signal having too short or too long waveforms over predetermined thresholds, so as to maintain the waveform lengths of the EFM signals between 3T and 11T.
The conventional approach obviously only monitors whether the waveform lengths of the EFM signal exceed 11T and fails to determine the accuracy of the EFM signal. Therefore, malfunctions may be arisen when the PLL adjusts the locked frequency based on the wrong EFM signal derived from incorrect data. For example, when the system clock SYS_CLK samples the EFM_DATA_C series, the first detected waveform length of the EFM signal longer than 11T will occur in the thirteen system clock periods Ti+1˜Ti+13 (indicated by numeral 106A). Next, the optical system will find that the waveform lengths of Tj+1˜Tj+12 (indicated by numeral 106B) and Tk+3˜Tk+15 (indicated by numeral 106D) in EFM_DATA_C are longer than 11T when the EFM_DATA_C series being continuously sampled. Based on the algorithm of the prior art scheme, because the waveform lengths longer than 11T in the EFM_DATA_C has reached three times (respectively indicated by 106A, 106B and 106D), the optical system will decrease its system clock frequency for the next detecting window. However, when looking closer to the EFM_DATA_C series, it is obvious that the waveform lengths in both Tk+2˜Tk+3 (indicated by numeral 106C) and Tk+16˜Tk+17 (indicated by numeral 106E) periods are only 2T, which obviously do not comply with the data storage specification for the optical disc. Namely, the currently read data might be error or damaged. At this time, if the optical system decreases its system clock frequency, the PLL might be suddenly slowed down, which may cause unnecessary resource and time costs.
Furthermore, the conventional scheme will terminate the PLL operation firstly when the optical system needs to seek tracks for accessing required data, until the pick-up head completes the track-seeking operation. Therefore, the data access will not be performed until the PLL becomes stable, which evidently enlarges operation time under seeking mode. As a result, there is a need to provide a method being capable of eliminating disadvantages of the traditional approach, which can avoid incorrect adjustment operations regarding system clock frequency caused by error data, and can also upgrade the performance under the seeking mode.